1. Field of the Invention
The present invention relates to an apparatus for processing semiconductor wafers at reduced pressure in a chamber, and a valve used therefor. In processes of manufacturing semiconductor integrated circuits, apparatuses which perform processing of semiconductor wafers placed in a vacuum chamber are widely used. A vacuum chamber used with such an apparatus is connected to an external exhaust system and has the function of maintaining a high degree of vacuum. Also, the vacuum chamber allows the introduction of the necessary source gases. Valves are used in the processing apparatus to control the pressure within the chamber and the introduction of gases. The present invention particularly relates to a processing apparatus having a valve inserted between the vacuum chamber and the exhaust system that is capable of accurately controlling the flow of gas at the start of a processing operation.
2. Description of the Related Art
In general, in a processing apparatus at reduced pressure, in addition to the so-called leak valve for keeping the interior of the chamber airtight against the directly contacting atmosphere, various valves are used between the chamber and an exhaust system for a low degree of vacuum, and between exhaust systems for low and high degrees of vacuum.
In general, the chamber, the interior of which is at atmospheric pressure, is exhausted in the following manner. First, the interior of the chamber is connected to a low-vacuum exhaust system, such as an oil-sealed rotary vacuum pump, so as to be evacuated to a degree of vacuum of the order of, e.g., 10.sup.-2 to 10.sup.-3 Pa. Subsequently, the connection of the chamber is switched by valves to a high-vacuum exhaust system, such as an oil or mercury diffusion pump or an ion pump of varying type, so as to achieve a high degree of vacuum of, e.g., 10.sup.-8 to 10.sup.-9 Pa, or sometimes, a very high degree of vacuum of, e.g., 10.sup.-10 Pa.
Between the chamber and the vacuum exhaust system connected therewith, a valve of varying type, such as a gate valve or lift valve, is used in accordance with the configuration of the associated pipes or the purpose of use. Among usable valves, a lift valve is particularly suitable for use in the main exhaust passage because it is possible to achieve a relatively large opening area and, hence, to effect efficient exhaust even when the degree of vacuum within the chamber has already risen and a low pressure prevails therein.
However, the action of opening or closing a lift valve is such that it either completely opens or completely closes. With this action, therefore, it is impossible to vary the sectional area of the valve opening and control the amount of exhaust accordingly.
If such a lift valve is disposed between the chamber of a processing apparatus and the vacuum exhaust system, this leads to the following problem. At the start of exhausting the interior of the chamber which is at atmospheric pressure, during the so-called rough pumping, upon the opening of the lift valve, a great difference in pressure occurs, whereby air within the chamber is rapidly discharged in such a manner that the air within the chamber is temporarily made turbulent.
Within a chamber of a processing apparatus, a lot of particles are generated by processing. In addition, complicated jigs, etc. are generally installed. Therefore, the interior of the chamber tends to be difficult to keep clean.
Although the particles may vary in accordance with the processing performed by the apparatus and the subject of the processing, the particles are usually small light particles such as fine pieces, fine powdery particles or scales from thin films which are made of various materials such as metals or dielectric materials.
As a result, in the chamber of a processing apparatus in which a conventional valve is used, particles, such as those described above, are stirred up when air is made turbulent during rough pumping.
The thus stirred-up particles may adhere to the surface of a substrate which is disposed within the apparatus and is to be processed. If the apparatus is a deposition or sputtering apparatus, particles may adhere to the evaporation source or target. These problems are serious because they can lower the yield of the process.